Method and apparatus for tracking a resonant frequency

ABSTRACT

An arrangement for tracking resonant frequency of electrically resonant structures through a single channel includes a variable frequency oscillator associated with each resonant structure which provides an excitation signal of a variable frequency encompassing a possible resonant frequency of the associated resonant structure. Coupling device(s) are provided which connect each variable frequency oscillator to said resonant structure(s). An I-mixer is provided for each oscillator which forms a synchronous detector, a first input of each I-mixer being connected to its associated oscillator and a second input being connected to the coupling device, each I-mixer mixing the excitation signal from the associated variable frequency oscillator with a response signal generated by the resonant structure(s) in response to each excitation signal. The output of each I-mixer is filtered to remove sum products of the excitation and response signals, thereby leaving an amplitude modulation component of the signal, which is processed in a control loop to track the resonant frequency of each resonant structure.

FIELD OF THE INVENTION

The present invention relates to improved methods for tracking resonantfrequencies of electrically resonant structures, in particularstructures, which are mounted remotely from the driving and sensingelectronics.

BACKGROUND OF THE INVENTION

International patent application no. WO 98/21818 discloses a system fortracking the resonant frequency of an electrically resonant structure inwhich a variable frequency oscillator, which generates an excitationsignal of a variable frequency encompassing the possible resonantfrequency range of the target resonant structure, is connected to theresonant structure by a bi-directional RF transmission line. Theproportion of the excitation signal energy reflected and the proportiondissipated by the resonant structure will depend upon the differencebetween the frequency of the excitation signal and the resonantfrequency of the resonant structure, and the transmission lineincorporates a directional coupler, which generates a directionalcoupler signal proportional to the reflected signal from the resonantstructure. The directional coupler signal is conditioned by a processorto provide a feedback signal to the input of the variable frequencyoscillator such that the mean frequency of the excitation signal iscaused continuously to track the varying resonant frequency of theresonant structure.

This arrangement has particular application in the non-contact torquemeasurement using SAW (surface acoustic wave) devices as the sensingelements. Many such applications, however, use two SAW devices attachedto a rotating shaft in such a way that when torque is applied oneresonator is put in tension whilst the second is put in compression.This causes the resonant frequency of the first device to reduce whilstthe second will increase. The two devices would normally have a nominaldifference between them of 1 MHz, such that with torque the output fromthe system is a difference frequency that changes about 1 MHz withapplied torque. However, in order to be used in conjunction with thetracking system of the prior art, the two sensors on the shaft must beelectrically connected to the stator of the assembly via two pairs ofnon-contacting rotary coupled transmission lines. The use of two pairsof couplers has the disadvantage that the size and complexity of themechanical assembly is increased, and thereby the cost. In addition, therotary coupled transmission line can load the SAW resonator and therebymodify its frequency. As the system is a differential one, if bothcouples modify their respective sensor response by the same amount, thenthis effect can be cancelled out, but if the two channels are notidentical then an error in the reading obtained can result.

The arrangement of the prior art has the further disadvantage that therequirement for a directional coupler increases the complexity of thearrangement.

SUMMARY

According to one aspect of the present invention there is provided amethod of tracking the resonant frequency of an electrically resonantstructure comprising the steps of exciting the resonant structure with areference signal of a variable frequency encompassing the possibleresonant frequency of the resonant structure, mixing a response signalfrom the resonant structure with the reference signal, filtering themixed response and reference signals to remove the sum products from thecomposite signal, and using the resulting amplitude modulation componentof the response signal within a control loop to track the resonantfrequency of the resonant structure.

The present invention further provides an apparatus for tracking theresonant frequency of an electrically resonant structure, comprising avariable frequency oscillator providing an excitation signal of avariable frequency encompassing the possible resonant frequency of saidresonant structure, coupling means connecting said variable frequencyoscillator to said resonant structure, an I-mixer forming a synchronousdetector having a first input connected to said oscillator and a secondinput connected to the coupling device, the I-mixer mixing theexcitation signal from the variable frequency oscillator with a responsesignal generated by the resonant structure in response to the excitationsignal, a filter connected to the output of the I-mixer which filtersthe output of the I-mixer to remove the sum products of excitation andresponse signals, thereby leaving just an amplitude modulation componentof the signal, and processing means which processes the filtered signalto track the resonant frequency of the resonant structure.

An apparatus for and method of tracking a resonant frequency inaccordance with the invention has the advantage that it enables multipleresonant structures to be connected together and interrogated through asingle channel, whilst, at the same time, obviating the need for adirectional coupler to be used.

Preferably, the reference signal from the oscillator is mixed, throughthe coupling means, with a second reference signal from a secondoscillator of a variable frequency encompassing the possible resonantfrequency of a second resonant structure, the first and second resonantstructures having a nominal difference frequency, and said first andsecond resonant structures are excited with said mixed signal. Thecomposite response signal of said first and second resonant structuresis then mixed with the first reference signal, the mixed signal filteredusing a filter, preferably a low pass filter, and the resulting signalused within a control loop to track the resonant frequency of the firstresonant structure, and it is also mixed with the second referencesignal, by a separate mixer, the mixed signal filtered and the resultingsignal used within a control loop to track the resonant frequency of thesecond resonant structure. In this way, it is possible to track theresonant frequency of a pair of structures connected in parallel througha single channel. The coupling of the two signals to the resonantstructures is preferably achieved by use of a summer connected to theinput of the coupling means.

An impedance such as a resistor is preferably provided between theoscillator and the resonant structure, in particular between theoscillator and the coupling means. Furthermore, the or each signalsource preferably has a low output impedance, which has the advantage ofsuppressing any amplitude modulation of the or each reference signal.

In a particularly preferred embodiment of the invention, a Q-mixer isprovided for the or each signal source, to one input of which isconnected the signal source through a phase shifter which shifts thereference signal received at the first input by preferably 90 degrees,and to the other input of which is connected the coupling means so as todeliver the response signal thereto. The output of the or each Q-mixeris similarly filtered to remove the sum products of the input signals,thereby leaving just the amplitude modulation component of the responsesignal. The square of that signal is then summed with the square of thefiltered signal output of the associated I-mixer and then processed totrack the resonant frequency of the associated resonant structure. Inthis way, errors in the tracked frequency resulting from the phase delayof the signal at the input of the coupling device are eliminated.

The squaring and summing operations may be carried out digitally by useof A/D converters and suitable digital processing means, which may alsocalculate first harmonic amplitudes of the demodulated signals producecodes for controlling the carrier frequency of the signal sources.Alternatively, analog signal squaring means may be utilized, such as amixer with its inputs linked.

It will, of course, be understood that the system may be expanded totrack the resonant frequencies of more than two structures, such as SAWdevices. For such multiple resonant structures, there is preferably anominal frequency difference between each device of at least 1 MHz.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be well understood, there will now bedescribed some embodiments thereof; given by way of example, referencebeing made to the accompanying drawings, in which:

FIG. 1 is a simplified schematic representation of a system embodyingthe invention suitable for tracking the resonant frequency of a singlestructure;

FIG. 2 is a schematic representation of a system embodying the inventionsuitable for tracking the resonant frequency of two resonant structures;and

FIG. 3 is a schematic representation of a system according to analternative embodiment containing two resonant structures with I and Qsynchronous detectors.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring first to FIG. 1, there is shown a simplified schematic of asystem embodying the invention. The output of a signal source 3 isconnected directly to one input 5 a of a mixer 5 and through a resistor4 to the input of a coupling device 2, which, in turn, is connected to asingle SAW resonator 1. The input of the coupling device 2 is alsoconnected to a second input 5 b of the mixer 5. The coupling devicepreferably takes the form of a rotational contactless coupler, but othercoupling devices are also possible, and the signal source 3 takes theform of a high frequency oscillator with a center frequency within thebandwidth of the resonator 1 and it is frequency modulated again withina deviation that is within the bandwidth of the resonator. Uponoperation of the system, the SAW resonator 1 is excited by the referencesignal from the signal source 3. The impedance of the SAW resonator 1will change rapidly with frequency around its resonant point and willform a potential divider with the resistor 4, so that when the impedanceof the resonator 1 is high compared with that of the resistor 4, therewill be minimal voltage drop across the resistor 4, whereas when theimpedance of the resonator 1 is low compared with that of the resistor4, there will be a large voltage drop across the resistor 4 and minimalacross the resonator 1. In this way, the amplitude of the response ofthe SAW 1 to the reference signal, as seen at the second input 5 b ofthe mixer 5 from the coupling device 2, will also vary as the frequencyof the signal source 3 is modulated. By using a signal source 3 of lowoutput impedance, the amplitude modulation of the output of the signalsource 3 fed to the first input 5 a of the mixer will be suppressed. Asa result, the mixer 5, acting as a synchronous detector, will outputthrough its output line 5 c a signal which will be the sum of thedriving signal from the signal source 3 and the amplitude modulatedsignal response of the resonator 1. A low pass filter 6 is then used toremove the sum products of the signals, leaving just the amplitudemodulation component of the signal which can then be used within acontrol loop to track the resonant frequency of the SAW device 1 in themanner described in WO98/21818.

In a development of this embodiment not illustrated, a buffer isinserted in front of the resistor 4 that further reduces the parasiticamplitude modulation in the reference signal.

FIG. 2 shows a schematic representation of how the system of FIG. 1 canbe used to connect together and interrogate two SAW resonators whichhave a nominal difference frequency of, say, 1 Mhz through a singlechannel which requires just a single coupling device. The two SAWdevices 11, 21 are arranged in parallel and connected to the couplingdevice 2 which is, in turn, connected to the output of a summer 20. Oneinput of the summer 20 is connected to one input 15 b of a first mixer15 and through a first resistor 14 to a first signal source 13, whichgenerates a first reference signal, the first signal source also beingcoupled directly to the other input 15 a of the first mixer 15. Theother input of the summer 20 is connected to one input 25 b of a secondmixer 25 and through a second resistor 24 to a second signal source 23,which generates a second reference signal, the second signal source alsobeing coupled directly to the other input 25 a of the second mixer 25.Each of the signal sources 13, 23 takes the form of a high frequencyoscillator with a center frequency within the bandwidth of itsassociated resonator 11, 21 and it is frequency modulated again within adeviation that is within the bandwidth of its associated resonator 11,21. The coupling device 2 again preferably takes the form of arotational contactless coupler, but other coupling devices are alsopossible.

Each half of the system then operates in the same manner as describedabove in relation to FIG. 1, with the reference signal from the firstsignal source 13 exciting the first SAW 11 and the reference signal fromthe second signal source 23 exciting the second SAW 21. The amplitudemodulated response signals from the two SAWs 11, 21 will then be fedback through the coupling device 2 and the combined signals fed to theinputs 15 b, 25 b of both the first and the second mixer 15, 25 by thesummer 20. At the first mixer 15, the combined response signal is mixedwith the reference signal from the first signal source 13 and the sumproducts are then removed by the first low pass filter 16 connected tothe output 15 c of the first mixer 15. Furthermore, because of thenominal difference frequency between the two SAWs and because theamplitude modulation caused by each SAW device 11, 21 will be at thesame fundamental and harmonic frequencies, the output of the first lowpass filter 16 can easily be processed electronically in a manner wellknown to the skilled person to separate out the amplitude modulatedcomponent of the response from the first SAW 11. This can then be usedwithin a control loop to the first signal source 13 to track theresonant frequency of the first SAW 11.

Similarly, at the second mixer 25, the combined response signal is mixedwith the reference signal from the second signal source 23 and the sumproducts are then removed by a second low pass filter 26 connected tothe output 25 c of the second mixer 25. The output of the second lowpass filter 26 can then be processed electronically to separate out theamplitude modulated component of the response from the second SAW 21 duein the nominal frequency difference between the first and second SAWS11, 21. This can then be used within a control loop to the second signalsource 23 to track the resonant frequency of the second SAW 21.

For example, if the SAW devices 11, 21 have nominal frequencies of 200MHz and 201 MHz, giving a nominal difference frequency of 1 MHz and theamplitude modulation caused by each SAW device is at 5 kHz with the2^(nd) harmonics at 10 kHz, these will be excited by the referencesignals produced by the two signal sources 13, 23 having frequencies of200 MHz FM and 201 MHz FM respectively. When the 200 Mhz FM signal ismixed with the composite 200 and 201 MHz FM response signal withamplitude modulation from the SAWs 11, 21, the difference product willbe the 5 kHz signal generated by the modulation due to the excitation ofthe 200 MHz SAW, the modulation caused by the 201 MHz device beingoffset by 1 MHz and therefore easily filtered out. Similarly, when the201 MHz FM signal is mixed with the composite response signal, themodulation caused by the 200 MHz SAW can also easily be filtered out.

A drawback with the embodiments described above in relation to FIG. 1and 2 is that the actual frequencies that will be tracked using thedemodulated signal produced by the synchronous detector will slightlydiffer from the resonant frequency of the SAW device and the amount ofthis difference will depend on the phase delay of the signal at theinput of the coupling device. In some cases it may be difficult toensure a high stability of the phase delay, which will result in randomerrors in the measurement of the resonant frequency. FIG. 3 shows athird embodiment of the invention in which phase delay effects areeliminated by mixing the reference and response signals for each signalsource in an IQ mixer and producing the demodulated signal as a sum ofthe squares of the signals at I (in-phase) and Q (Quadrature) outputs ofthe IQ mixers 30, 40. This is achieved by supplementing the system ofFIG. 2 with an additional pair of Q-mixers 32, 42, one associated witheach signal source 13, 23.

One input of the first Q-mixer 32 associated with the first signalsource 13 is connected to the first signal source 13 through a 90 degreephase shifter 31 so as to receive a phase shifted version of thereference signal from the first signal source 13. The other input of thefirst Q-mixer 32 is connected to the summer 20 so as to receive theresponse signals from the two SAW devices 11, 21. The output of thefirst Q-mixer 32 is then filtered using a low pass filter 33 beforebeing squared and then summed with the filtered and squared output ofthe first mixer 15, which is based in the in-phase reference signal. Thesum of the squares of these two signals will not, then, depend on thephase delay of the input signal.

The squaring and summing of the signals may be achieved by analog meansusing looped mixers 34, 35 and a summer as shown in FIG. 3.Alternatively, the output of the low pass filters 16, 33 can beconverted into digital signals using A/D converters and the squaring andadding of the signals performed by a digital processor. Apart fromperforming the squaring and adding functions, the digital processor willalso calculate the first harmonic amplitudes of the demodulated signalsand produce the codes that will control the carrier frequencies of thedigital synthesizers used as the signal sources.

It will, of course, be understood that a second Q-mixer 42, phaseshifter 41 and low pass filter 43 will be associated with the secondsignal source 23.

1. A method of tracking a resonant frequency of an electrically resonantstructure comprising the steps of exciting the resonant structure with areference signal of a variable frequency encompassing the possibleresonant frequency of the resonant structure, mixing a response signalfrom the resonant structure with the reference signal, filtering themixed response and reference signals to remove the sum products from thecomposite signal, using the resulting amplitude modulation component ofthe response signal within a control loop to track the resonantfrequency of the resonant structure, summing the reference signal fromsaid oscillator with a second reference signal of a variable frequencyencompassing the possible resonant frequency of a second resonantstructure, the first and second resonant structures having a nominaldifference frequency, exciting said first and second resonant structureswith said mixed signal, mixing the composite response signal of saidfirst and second resonant structures with the first reference signal,filtering the mixed signal and using the resulting signal within acontrol loop to track the resonant frequency of the first resonantstructure, and mixing the composite response signal of said first andsecond structures with said second reference signal, filtering the mixedsignal and using the resulting signal within a control loop to track theresonant frequency of the second resonant structure.
 2. (canceled)
 3. Amethod according to claim 1, wherein the or each mixed response andreference signals are filtered using a low pass filter.
 4. A methodaccording to claim 1, comprising the further step of suppressing theamplitude modulation of the or each reference signal by using a signalsource of low output impedance.
 5. A method according to claim, whereinthe or each reference signal passes through an impedance before excitingthe or each resonant structure.
 6. A method according to claim 1,comprising the further step, for the or each reference signal, of mixingthe response signal with a phase shifted version of the or eachreference signal, filtering said mixed signal, squaring the filteredin-phase and phase shifted mixed response and reference signals, summingthe associated squared signals and using the result within a controlloop to provide a phase compensated track of the resonant frequency ofthe associated resonant structure.
 7. A method according to claim 6,wherein the reference signal is phase shifted through 90 degrees.
 8. Anapparatus for tracking a resonant frequency of an electrically resonantstructure, comprising a variable frequency oscillator providing anexcitation signal of a variable frequency encompassing the possibleresonant frequency of said resonant structure, coupling means connectingsaid variable frequency oscillator to said resonant structure, anI-mixer forming a synchronous detector having a first input connected tosaid oscillator and a second input connected to the coupling device, theI-mixer mixing the excitation signal from the variable frequencyoscillator with a response signal generated by the resonant structure inresponse to the excitation signal, a filter connected to the output ofthe I-mixer which filters the output of the I-mixer to remove the sumproducts of excitation and response signals, thereby leaving just anamplitude modulation component of the signal, and processing means whichprocesses the filtered signal to track the resonant frequency of theresonant structure, wherein the apparatus for tracking the resonantfrequencies includes a pair of electrically resonant structures having anominal difference frequency and further comprising a second variablefrequency oscillator connected to the coupling means, said firstvariable frequency oscillator providing an excitation signal of avariable frequency encompassing the possible resonant frequency of thefirst resonant structure and said second variable frequency oscillatorproviding an excitation signal of a variable frequency encompassing thepossible resonant frequency of the second resonant structure, a secondI-mixer forming a synchronous detector associated with the secondoscillator having its first input connected to the second oscillator andits second input connected to the coupling device so as to mix theexcitation signal from the second oscillator with a composite responsesignal received from said first and second resonant structures, and asecond filter connected to the output of the second I-mixer whichfilters the output signal.
 9. (canceled)
 10. An apparatus according toclaim 8, wherein said first and second resonant structure are connectedin parallel.
 11. An apparatus according to claim 8, further including asummer having first and second inputs connected to the first and secondoscillators respectively, and an output connected to the coupling means.12. An apparatus according to claim 8, wherein the or each filter is alow pass filter.
 13. An apparatus according to claim 8, furthercomprising an impedance connected between the or each oscillator and thecoupling device, the first input of the or each I-mixer being connectedbetween its associated oscillator and its impedance and the second inputof the or each I-mixer being connected between the associated impedanceand the coupling device.
 14. An apparatus according to claim 8, furtherincluding a Q-mixer associated with the or each oscillator having afirst input connected to its associated oscillator by means of phaseshifting means and a second input connected to the coupling means suchthat the or each Q-mixer mixes a phase shifted version of the excitationsignal from its associated oscillator with the response signal, a filterconnected to the output of the or each Q-mixer which removes the sumproducts of the phase shifted excitation and response signals, so as toleave just an amplitude modulation component of the signal, and furtherincluding means associated with the or each oscillator for squaring andthen summing the filtered signals from the I- and Q-mixers associatedwith the or each oscillator, said processing means processing the sum ofthe squares of the filtered signals from said I- and Q-mixers, wherebyphase delay effects are eliminated.
 15. An apparatus according to claim14, wherein the or each phase shifting means phase shifts the signal by90 degrees.
 16. An apparatus according to claim 14, wherein said meansfor squaring and summing said signals comprises first analog signalsquaring means connected to the filtered output of the or each I-mixer,a second analog squaring means connected to the filtered output of theor each Q-mixer, and a summer associated with the or each pair of I andQ mixer having a first and second inputs connected to the outputs of theassociated first and second squaring means.
 17. An apparatus accordingto claim 16, wherein the said analog signal squaring means each comprisea mixer having first and second inputs connected together to the outputof its associated filter.
 18. An apparatus according to claim 14,wherein said means for squaring and summing the signals comprises adigital processor, the output of each filter being connected to ananalogue to digital converter which is, in turn, connected to an inputof the digital processor.
 19. An apparatus according to claim 18,wherein the or each digital processor also calculates first harmonicamplitudes of the demodulated signals and produces codes for controllingthe carrier frequency of the signal source.
 20. An apparatus accordingto claim 8, wherein the coupling means is a rotational contactlesscoupler.